Hexafluoroethane (CF3CF3) is used, for example, as a cleaning gas or etching gas for semiconductors. Several processes have conventionally been known for the production of CF3CF3, such as
(1) electrolytic fluorination processes using ethane and/or ethylene as the starting material,
(2) thermal decomposition processes involving thermal decomposition of tetrafluoroethylene,
(3) processes in which acetylene, ethylene and/or ethane are fluorinated using metal fluorides,
(4) processes in which dichlorotetrafluoroethane or chloropentafluoroethane is fluorinated using hydrogen fluoride in the presence of a fluorination catalyst, and
(5) direct fluorination processes involving reaction of tetrafluoroethane and/or pentafluoroethane with fluorine.
However, when the process of (4) above is used, for example, the resulting CF3CF3 contains impurities in the form of compounds derived from the starting material or compounds newly produced by the reaction. Particularly problematic among such impurities are chlorine compounds, which are difficult to separate from CF3CF3.
When the process of (5) above is used, for example, the resulting CF3CF3 also contains impurities in the form of compounds derived from the starting material or compounds newly produced by the reaction. These impurities also include chlorine compounds which are difficult to separate from CF3CF3. Purification may be carried out before reaction with fluorine gas in order to reduce the chlorine compounds in the starting material, but industrial application of conventionally known purification processes is difficult in most cases.
The chlorine compounds in CF3CF3 include compounds such as chloromethane, chlorodifluoromethane, chlorotrifluoromethane, chloropentafluoroethane, dichlorotetrafluoroethane, chlorotetrafluoroethane, chlorotrifluoroethane, chlorotrifluoroethylene and the like.
Among these chlorine compounds, chlorotrifluoromethane is difficult to separate because it forms an azeotropic mixture with CF3CF3. Examples of methods of purifying CF3CF3 containing chlorotrifluoromethane include the method described in U.S. Pat. No. 5,523,499 in which CF3CF3 containing impurities such as trifluoromethane (CHF3) and chlorotrifluoromethane (CClF3) is contacted with an adsorbing agent such as active carbon or a molecular sieve to adsorb and remove the impurities.
Purification methods using such adsorbing agents require special equipment for regeneration of the adsorbing agents at roughly consistent intervals in the case of constant operation, although this depends on the impurity content. For example, in a process wherein two adsorption towers are provided and operation alternately switches between a step of adsorption of impurities and a step of regeneration of the adsorbing agent, large amounts of gas may be continuously treated, but the adsorbed and removed chlorotrifluoromethane cannot be directly released into the atmosphere since it is a specific freon implicated in destruction of the ozone layer, and therefore some means must be used to render it harmless.
On the other hand, pentafluoroethane (CF3CHF2) is used, for example, as a low-temperature refrigerant, or as a starting material for production of hexafluoroethane (CF3CF3). As examples of conventionally known processes for production of pentafluoroethane there may be mentioned the following:
(1) a process in which perchloroethylene (CCl2═CCl2) or its fluorinated product is fluorinated with hydrogen fluoride (Japanese Unexamined Patent Publication No. 8-268932, Japanese Unexamined Patent Publication No. 9-511515),
(2) a process in which chloropentafluoroethane (CClF2CF3) is subjected to hydrogenolysis (Japanese Patent No. 2540409), and
(3) a process in which fluorine gas is reacted with halogen-containing ethylene (Japanese Unexamined Patent Publication No. 1-38034).
When such processes are used, the major impurities in the target product, pentafluoroethane, are chlorine compounds which of course contain chlorine atoms in the molecules. Examples of such chlorine compounds include compounds with one carbon atom such as chloromethane, chlorodifluoromethane and chlorotrifluoromethane, compounds with two carbon atoms such as chloropentafluoroethane, dichlorotetrafluoroethane, chlorotetrafluoroethane and chlorotrifluoroethane, and unsaturated compounds such as chlorotrifluoroethylene.
When hexafluoroethane is produced by direct fluorination reaction with pentafluoroethane and fluorine gas (F2), any of these chlorine compounds included in the pentafluoroethane will react with the fluorine gas, producing chlorine, hydrogen chloride, chlorine fluorides, or various chlorofluorocarbons. While perfluorocarbons (PFCs) included in the pentafluoroethane do not present any notable problem, chloromethane (CH3Cl) and chlorodifluoromethane (CHClF2) will react with fluorine gas to produce chlorotrifluoromethane (CClF3). Hexafluoroethane and chlorotrifluoromethane form an azeotropic mixture, and therefore removal of chlorotrifluoromethane is difficult even by distillation or adsorption purification. Thus, when pentafluoroethane and fluorine gas are reacted to produce hexafluoroethane, it is preferred to use pentafluoroethane with a minimal chlorine compound content.
With conventional processes for production of pentafluoroethane, the chlorine compound content in the pentafluoroethane can be as high as about 1 vol % in total. Repeated distillation has therefore been considered to remove the chlorine compounds in the pentafluoroethane and thus increase its purity, but because of the increased production cost of the distillation, as well as distillation loss, this procedure has been uneconomical, while some of the chlorine compounds also form azeotropic mixtures or azeotrope-like mixtures with pentafluoroethane and create a situation in which it very difficult to separate the chlorine compounds by distillation procedures alone. In particular, chloropentafluoroethane (CClF2CF3) is usually included in pentafluoroethane at a concentration of a few thousand ppm or greater, but since pentafluoroethane and chloropentafluoroethane form an azeotropic mixture, they are difficult to separate by distillation, which is the commonly employed separation and purification method.